Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photodetector, being integrated within a pickup. The photodetector is used for detecting reflected light of the laser beam when reading data on the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to about 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm and a numerical aperture of 0.85 is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T or 9T is used, where T is the channel bit length and wherein 2T corresponds with a minimum mark length of 138, 149 or 160 nm.
The diffraction limit of optical instruments as described by the Abbe theory is about lambda/2NA, which is 238 nm for a Blu-Ray type pickup with a wavelength lambda=405 nm and a numerical aperture NA=0.85. This theoretical minimal detectable length from the diffraction theory is corresponding to a period of the pattern function, which is formed of a pit and of a land having the same length. The smallest detectable element of such a system is a pit or a land having a length of about lambda/4NA, which corresponds for a Blu-Ray type pickup with a length of 120 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and which center reflectivity is dependent on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the optical resolution limit of lambda/4NA of a corresponding pickup.
The nonlinear layer is often called a super-resolution near-field structure (Super-RENS) layer because it is assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. Super-RENS optical discs comprising a super resolution near-field structure formed of a metal oxide, a polymer compound or a phase-change layer comprising a GeSbTe or AgInSbTe are known.
An optical disc comprising a substrate layer, a read-only data layer having a data structure including user data and control data arranged in tracks on the substrate layer, and a nonlinear layer disposed on the data layer wherein the control data include a protection code is known from EP-A-2136363.